To ensure the reliability of transmitted information in wireless communication systems, error-detection and error-correction techniques are often employed. One such technique is known as hybrid automatic transmission request (HARQ), which integrates both forward error correction (FEC) coding and an automatic transmission request (ARQ) scheme for error-control. Forward error correction is a coding technique that provides error-correction capability by way of redundancy. Automatic transmission request is a method for controlling errors by retransmitting information. Hybrid ARQ (HARQ) may be implemented in various ways, one of which is based on a stop-and-wait ARQ protocol, in which a transmitter provides retransmissions in response to a negative acknowledgment (NACK) generated by the receiver. HARQ uses each transmission advantageously by performing retransmission combining using each transmitted packet, which are stored at the receiver.
HARQ often requires a large buffer at the receiver side to store soft information (such as log-likelihood ratios, or LLRs) generated from previous transmissions. The soft information is typically stored in its entirety in order to achieve optimal retransmission combining performance. Specifically, the same number of soft-bits that are used for FEC are also generally stored in memory for HARQ combining purposes, regardless of whether the soft information corresponds to the initial transmission or a retransmission. Accordingly, designers are faced with the undesirable choice of storing a smaller number of soft-bits per symbol, which results in a performance loss but requires a smaller buffer, or using a larger number of soft-bits per symbol, which results in a performance gain but requires a large external memory and/or an increase in memory bandwidth requirements. Neither of these design choices is particularly attractive.